Half-bridge and full-bridge circuits are known as a circuit arrangement for generating an AC voltage from a DC voltage, also referred to below as an inverter. Half-bridge circuits are used in particular for operating gas discharge lamps.
The half-bridge contains two series-connected electronic switches which are closed and opened alternately. These switches are driven either from a control circuit or from a connected load circuit. In the latter case, the half-bridge itself drives the electronic switches using feedback means, for which reason a circuit arrangement such as this is referred to as a self-oscillating half-bridge. In the prior art, an inexpensive way of implementing an inverter is to use a self-oscillating half-bridge having bipolar transistors. This eliminates the need for a control circuit and makes it possible to use inexpensive bipolar transistors.
The specification U.S. Pat. No. 5,563,777 (Miki) discloses various embodiments for self-oscillating half-bridges. The feedback means used is a transformer, whose primary side is arranged in the load circuit and whose secondary side drives the electronic switches.
An electronic switch generally has two make contacts and a control contact. A load resistor may be defined between the make contacts, and a control resistor may be defined between a make contact and the control contact. In the case of a bipolar transistor in a half-bridge, the emitter and the collector form the make contacts, and the base forms the control contact. The control resistor is positioned between the base and the emitter. In the case of a MOSFET in a half-bridge, the source and the drain form the make contacts, and the gate forms the control contact. The control resistor is positioned between the gate and the source.
The specification U.S. Pat. No. 5,563,777 (Miki) shows a number of exemplary embodiments for the transformer. Firstly, the transformer may be in the form of a separate transformer which acts only as the feedback means. This transformer may be either saturated or unsaturated. Secondly, the transformer may be formed from an inductor in the load circuit, to which the secondary windings are applied. The inductor in the load circuit then forms the primary winding of the transformer. In applications for operating gas discharge lamps, this inductor may be used as the so-called lamp inductor. In other applications, it may be used, for example, to make near-resonance operation possible. As is also the case for the separate transformer mentioned above, the transformer which comprises the inductor may be saturated or unsaturated in design.
All of the embodiments known from the prior art have secondary windings which are connected in parallel with the control resistor.
The embodiments from the prior art have the following disadvantages: Embodiments with an unsaturated transformer sometimes have high switching losses since closing of the electronic switches is not always ensured when no voltage is applied. There are also sometimes high driving losses since the base currents of the electronic switches may be high in value.
Embodiments having a saturated transformer have high transformer losses owing to its high drive level. In addition, the saturation properties are subject to high manufacturing tolerances. This means that a complex selection process is required when selecting the mass-produced transformer.